We know the age-old myth (which may or may not have science behind it) that when women spend a lot of time around their gal pals, their menstrual cycles sync up. But turns out there's much more science behind the synchronization of another bodily function — the immune system.

According to a study recently published in the journal Nature Immunology, married couples' immune systems tend to look quite a lot alike.

For the study, a team at the University of Leuven took the blood samples of 670 people and tracked around 150 of those people — including 70 married couples — over six months, observing how their immune systems responded to their environment.

They had two major discoveries. The first one was that the immune systems of men and women of the same age don't seem to differ all that much. And the second, perhaps more interesting one was that the immune systems of married couples showed 50% less variation than randomly paired men and women of a similar demographic.

But how could this be? Could the couples somehow be distant blood relatives?

Well, fortunately, no. Past studies have shown that our environment, more than our genetics, plays the lead role in shaping our immune systems.

And our environments are made up of a litany of factors: diet, exercise routines, infections, air pollution — and we share much of those things with the people we live with. We eat together, drink together, go to the gym together, snuggle together, and go to IKEA together.

Which explains why the researchers also found that married couples also seem to have similar microbiomes — especially with all the bacteria we exchange just by kissing and holding hands.

So, the next time you exchange keys with someone, remember that you're also exchanging your gut bacteria. Love is just like a fairytale!